1. Field of the Invention
This invention relates to a polymeric composition based on tetraflyoroethylene (TFE) and ethylene (E) or chlorotrifuloroethylene (CTFE) and ethylene, which composition has sufficient flame resistance, drip resistance, and low smoke evolution to pass the Underwriter's Laboratory (UL) 910 Steiner Tunnel Test when burned in the form of insulation on wires which are bundled into a 25-pair jacketed cable. The composition is useful as insulation in plenum cable.
2. Background
In commercial buildings, the space between the false ceiling and the floor above is called a plenum and is commonly used for ventilation. If the plenum contains flammable materials, it can cause rapid spread of any fire that starts in the building. For this reason, regulations for the flame spread and smoke evolution of materials, such as communication cables, which are present in the plenum have become much more stringent in certain localities. It is now required in many locations that these cables pass the UL 910 "Test Method for Fire and Smoke Characteristics of Electrical and Optical Fiber Cables used in Air-Handling Spaces".
In some instances, the insulation on the wires in these cables is made of polyvinylidene fluoride (PVDF), which may provide difficulties in extrusion, both in startup and in long-term operation at the high temperatures required for extrusion. There is a need, therefore, for an insulation composition having good extrusion performance and good color stability at extrusion temperatures and which also gives the end-use jacketed cable a combination of flame resistance, low smoke evolution, and no flaming drips which can spread the fire.
Known copolymers of tetrafluoroethylene and ethylene (ETFE) are easy to extrude. Such known ETFE copolymers commonly contain a small amount of another fluoromonomer to provide for good physical properties at high temperatures. U.S. Pat. No. 4,677,175 discloses that the flame retardant properties of ETFE copolymers are better when the ethylene content is lower, but Comparative Example B provided herein below shows that this technology alone does not enable a cable to pass the UL 910 test.
U.S. Pat. No. 4,155,823 discloses that TiO.sub.2 or Sb.sub.2 O.sub.3 plus TiO.sub.2 may be used in crosslinked ETFE, with flame retardant properties being ascribed to the Sb.sub.2 O.sub.3. However, the discovery of an easily extrudable, ETFE insulation composition which enables a jacketed 25-pair cable to pass the UL 910 test has not been disclosed.
U.S. Pat. No. 3,005,795 (commonly assigned) discloses the use of PTFE as a way of increasing the melt viscosity of thermoplastics, and this material has been used as a drip suppressant in a thermoplastic cable jacket, as in U.S. Pat. No. 4,708,975.